1. Field of the Invention
The present invention generally relates to image forming apparatuses, image forming methods, and image forming program products, and more specifically, to an image forming apparatus such as a copier or a printer configured to form an image on a transferring member such as a recording medium, an image forming method, and an image forming program product.
2. Description of the Related Art
In recent image forming apparatuses such as color copiers or color printers, a color image is output with output velocity the same as that in a case of outputting monochrome images. Therefore, tandem type image forming apparatuses where toner images of plural colors are simultaneously formed have been mainly used.
In the tandem type image forming apparatuses, a photo sensitive body and a developing device of each color are individually provided. Toner images formed on the photo sensitive bodies of respective colors are transferred onto an intermediate transferring belt so that a color image is formed.
On the other hand, an image forming apparatus has been known having a structure where image forming parts of the monochrome image and the color image are separated in order to reduce the output time of the monochrome images in an environment where a proportion of outputting the monochrome images is high.
In the image forming apparatus, in a case where the monochrome image is formed, a black toner image is directly transferred onto a recording sheet so that the monochrome image is formed. In addition, in a case where the color image is formed, after the toner images of the respective colors formed on the belt are transferred onto the recording sheet, the black toner image is transferred onto the recording sheet where the color toner images have been transferred, so that a color image is formed. See, for example, Japan Laid-Open Patent Application Publication NO. 2004-205943.
The above-mentioned image forming apparatus where the image forming parts of the monochrome image and the color image are separated includes an intermediate transferring belt configured to form images of, for example, three colors and a conveyance belt configured to convey the recording sheet where the monochrome image is transferred.
In the above-mentioned image forming apparatus including plural belts, first, a monochrome image is transferred onto the recording sheet situated on the conveyance belt. Then, the images of three colors formed on the intermediate transferring belt are secondarily transferred onto the recording sheet where the monochrome image has been formed, so that the color image is formed.
In a case where a color matching process is performed where a color shift among plural colors is adjusted in the image forming apparatus, according to the related art, for example, first, a four color toner pattern of three chromic colors and one black color are formed on the conveyance belt. A toner pattern of four colors is detected by a sensor so that an amount of shift from a correct position of each color is calculated. In addition, the calculated shift amount is corrected by adjusting the timing of writing positions.
However, it is necessary for a four color toner pattern to be formed on the conveyance belt by using two belts, namely the intermediate transferring belt and the conveyance belt. Accordingly, in a case where belt velocities of these belts are different from each other, although the same length toner pattern is output, not only a shift of a writing position but also a shift in a toner pattern length direction may be generated in the formed toner pattern.
Because of this, in a case where the color matching process is performed, the shift of the toner pattern length due to the difference of the velocities may not be corrected by the correction of the timing of the writing position. Hence, it may not be possible to securely perform color matching of the shift among the colors.